Checking Radiation Levels Beyond the Immediate Bomb Shelter Area

Sometime no later than 24 – 30 hours after fallout has begun to come down, you (the RM) should take the survey meter and check the radiation levels in rooms next to the bomb shelter area and on the way to the outside. The purpose of this exploration is to:

1. Get an idea how dangerous the levels are outside the immediate bomb shelter area.

2. Estimate the risks in emergency operations.

3. Forecast when people could leave the bomb shelter for short periods and when they could move to safer areas if needed.

Your experience and training make you very valuable to the occupants of the bomb shelter. You should very carefully monitor your own exposure and make forecasts on future exposures so you will not exceed the limit of exposure set in Row A of the Penalty Table.

If you used the time-averaging method to find the safest location in the bomb shelter and the fallout pattern hasn’t been shifted by wind or rain, you may use the results of those measurements to estimate the radiation levels in the other rooms which you checked, by using the Ratio Method.

Suppose you stayed near location 1, your “home base”, during the first 24 hours after fallout arrived. Now you want to find out how high the radiation level is at location 5. Suppose you included location 5 in your time-average comparison. Then you can estimate the present reading at location 5 by first finding the ratio of the time-average reading at location 5 to the time-average reading at location 1. Then multiply this ratio times the current reading at location 1 to get the current reading at location 5. In other words,

Current read = (Time-avg reading at location # 5) x Current reading at location # 5 (Time-avg reading at location # 1) at location # 1

The measurements at the apartment bomb shelter will be used as an example. Suppose we would like to know what the survey meter reading wuold be at location 5 in the apartment bomb shelter at 2000 hr on July 5, without actually taking the survey meter to the location. We have been making measurements regularly at location 1, as shown in Radiation Exposure Record (above). We have the set of time-average measurements that were made earlier for seven locations, including locations 1 and 5, as listed in the Time-Averaging Table (above).

To get the current reading (at 2000 hr) at location 5 without taking a survey meter to that location, the following steps are taken:

1. The current reading (at 2000 hr) at location 1 is found to be 6.0 R/hr.

2. The time-average reading at location 5 was 1.435 R/hr.

3. The time-average reading at location 1 was 0.575 R/hr.

4. The ratio of the time-average reading at location 5 to the time-average reading at location 1 is 1.435/0.575 = 2.5.

5. The current reading at location 5 is estimated by multiplying the ratio obtained in setp 4 times the current reading at location 1, which yields 2.5 x 6.0 = 15 R/hr.

If more than one set of time-averaging measurements has been made, be sure to calculate the ratio with readings that were made in the same set of measurements.

Once the ratio of the time-average readings has been caluclated, that same ratio can be used to estimate the reading at the remote location at any other time, assuming that the fallout pattern hasn’t been shifted by rain or wind.

For example, the estimated reading at location 5 in the apartment bomb shelter at 2100 hr would be 2.5 times the reading at location 1 at that time, which is 5.5. The estimated reading at location 5 at 2100 hr would be 2.5 x 5.5 = 13.75 R/hr.

You may use the ratio method to estimate the radiation levels; first, at various strategic locations inside your bomb shelter building and, later, at various locations outside your building. First take a reading at your home-base location. Then take the survey meter (wear a dosimeter) to the strategic location and take a reading there. You will not need to use the time-average method after 24 hours after the last particles of fallout have arrived because the radiation levels will be decreasing slower than 1 percent per minute. The ratio of the reading at the strategic location to the reading at the home base can be used to estimate readings at the strategic location by multiplying that ratio times the home-base readings.

As an example, the RM at the apartment bomb shelter measured 2.1 R/hr at location 1, the home base, at 1000 hr on July 6. The Rm took the survey meter up the stairs and made a quick trip into the lobby of the apartment bomb shelter, where the survey meter reading was 85 R/hr. The ratio of the lobby to home-base reading was 40. By 1000 hr on July 7, the home-base reading was 1 R/hr. The ratio of 40 was used to estimate that the radiation level in the lobby at that time was 40 R/hr.

At that time the RM took the survey meter upstairs and out to the street in front of the apartment bomb shelter, where he measured a radiation level of 105 R/hr. His dosimeter showed an increase of 2 R for this trip, which he made as quickly as possible. The street to home-base ratio of readings was thus determined to be 105.

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